This invention is concerned with a method of manufacturing plain bearings and with materials and bearings manufactured by such methods.
Many types of plain bearings have a bearing surface layer which contains a high proportion of a fluoro-polymer such as polytetrafluoroethylene (PTFE), since such polymers have good low friction properties. Such bearing surface layers are inherently weak so that they are applied to a supporting substrate which is normally made of metal. However, as fluoro-polymers are difficult to secure to a substrate, in one common type of bearing, an intermediate layer is included which is usually made of sintered metal, eg bronze. In manufacturing this type of bearing, the PTFE is mixed with solvent, and fillers, to form a xe2x80x9cmushxe2x80x9d which is paste-like and is spread over the surface of the sintered metal before being forced into the interstices of the sintered metal so that the bearing surface layer material is bonded to the sintered metal. Because of the necessity to remove the solvent, the material can only be formed with a relatively thin surface layer (about 50 microns or less) over the sintered metal otherwise solvent induced blistering occurs. The thickness of the surface layer produced above the sintered bonding layer is also affected by the fact that the mush is rolled into the interstices of the sinter and to achieve sufficient impregnation to prevent the fluoro-polymer from delaminating from the backing in use, a relatively high pressure may be required which results in a low layer thickness above the sintered bonding layer. Because the surface layer is relatively thin, it cannot normally be machined to improve the surface quality or to perform a sizing operation to form a bearing having a precise wall thickness and bore dimension.
EP-A-0708892 describes a bearing made from a material comprising a metal backing having a sintered layer thereon and a bearing material lining comprising polytetrafluoroethylene and from 2 to 10 volume % of fibrillated aramid fibres infiltrated into the pores of the sintered bonding layer. This document also describes making a sheet of the bearing material by a paper-making type route, fusing and bonding the sheet by heat and pressure to the sintered bonding layer. However, the layer thickness above the sintered bonding layer on the backing material is again only 25 xcexcm.
WO97/06204 describes the manufacture of a bearing material in sheet form comprising an aromatic polyamide pulp and a fluororesin as main constituents. The method of manufacture is by a paper-making type process. However, no mention is made of any method of attaching the material so formed to a metal backing. Indeed, thicker material is obtained by forming a stack of a plurality of layers of the sheet material and sintering them together under an applied pressure. However, this results in only a monolithic block of the material from which a bearing may be formed by machining for example but which lacks the strengthening effect of a supporting metal backing where thin bearing layers or wall thickness are required.
It is an object of the present invention to provide a method of manufacturing a plain bearing based on a fluoro-polymer, which method enables a thicker bearing material layer above any supporting or bonding metal layer to be formed.